Tension Leg Platforms or “TLPs” have been used for some time in the offshore production of oil and gas. Often a tension leg platform will be moored by groupings of tendons at each corner of a platform. The lifetime of a given platform may be anywhere from 20-50+ years and thus it has become necessary to put systems in place to monitor the tendon tensions of platforms to ensure a platform can continue reliable and safe operations. Over time, a few different configurations of tendon tension monitoring have become the solution of choice for offshore TLPs. Each configuration involves the use of load measurement units and sensors. One configuration uses in-line sensors while the other configuration uses porch-mounted sensors. The in-line system type is often installed as part of the tendon string, whereas the porch-mounted system type is not part of the tendon but instead pre-installed as part of the tendon top connection apparatus. An in-line tendon tension monitoring system (“TTMS”) will often be configured to employ load measurement units consisting of relatively long sections of tendon pipe or forged tubing, and are connected in-line near the top of the tendon body. In a known configuration of an in-line system variable reluctance measurement technology (“VRMT”) sensors can be configured and mounted on the facing flanges that are attached to the outside of the load measurement section of the in-line system.
In contrast to an in-line system, a porch-mounted tendon tension monitoring system (“TTMS”) is often configured with three or more compression type load cells that are arranged about the opening in the tendon top connector assembly (“TCA”) and between bearing plates positioned either above or below a top connector flex element.
Over time it has become apparent that the variable reluctance measurement technology sensors configured on in-line tendon tension monitoring systems are significantly more reliable than the load cell sensors that are often configured in porch-mounted tendon tension monitoring systems. The initial cost of an in-line system far surpasses the initial cost of a porch-mounted system. However, conventional porch-mounted systems do not provide service life beyond five to ten years, and there are significant additional costs to servicing conventional porch-mounted systems as tension must be removed from the tendon being serviced. It would thus be desirable to configure the more reliable variable reluctance measurement technology sensor in a porch-mounted system. One problem with such a setup is that conventional variable reluctance measurement technology sensors have insufficient displacement sensitivity to be used as part of a porch-mounted system. There are also space considerations because of the limited space available between the tendon porch and the tendon top connector assembly. Conventional variable reluctance measurement technology sensors also do not fit the space profile a typical porch-mounted system would require.
Regarding variable reluctance measurement technology sensors, several prior art documents disclosed in-line VRMT sensor designs, these include U.S. Pat. Nos. 7,493,827, 6,752,039, and 6,422,089; and U.S. patent application Ser. No. 10/848,525. Mechanical amplifier systems and methods were further disclosed in several prior art documents, including U.S. Pat. No. 6,880,408, U.S. Pat. Appl. Ser. No. 60/375,789, and PCT App. No. PCT/US03/12869. Load monitoring systems and methods have also been disclosed in prior art documents, including U.S. Pat. No. 6,748,809, U.S. patent application Ser. Nos. 10/848,600 and 11/152,303, and PCT App. No. PCT/US03/15974.
Regarding VRMT sensor designs, prior art U.S. Pat. No. 7,493,827, which is incorporated herein by reference in its entirety, describes a VRMT sensor as a sensor that uses opposing magnetic cores contained in a support tube. Each of the magnetic cores is attached to opposing ends of the support tube. Thus, as the support tube expands along the tube axis, the ends of the support tube, which are perpendicular to the tube axis, separate. A magnetic circuit is formed having an inductance defined by the size of the gap between the magnetic cores. Accordingly, when the magnetic cores attached to the tube ends separate, the size of the gap between the magnetic cores is increased. Thus, when the inductance is altered, the amount of expansion that has occurred can be determined. Knowing the elastic characteristics of the support tube material, the amount of force applied to the support tube can be calculated. Similarly, contraction of the support tube results in a change in inductance that is indicative of the amount of stress reduction. Alternatively, the support tube can have very little stiffness relative to the structure that it is mounted on so that no load passes through the support tube and it merely displaces the same amount as the structure displaces in the region between the attachment points. The combination is tested under known loads to provide the calibration.
U.S. Pat. No. 7,493,827 goes on to state that one of the magnetic cores is generally preferred to be configured in a C-shape, and attached to an end plate by way of a bracket. The end plate may be one of the tube ends, or another plate that is in turn attached to the support tube. The C-shape is preferred for one of the magnetic cores so that the windings can be placed at the ends of the C-shaped cores. The other magnetic core is preferably I-shaped, and is attached to a second end plate by way of a second bracket. The second end plate, like the first end plate, may be the other tube end, or another plate that is in turn attached to the support tube. Thus, a cavity within the support tube containing the sensor is formed. Preferably, the cavity containing the sensor is sealed in a manner to prevent water or other damaging agents from entering the cavity and damaging the sensor or its wiring. The cavity can also be filled with a low durometer elastomeric potting material, silicon oil, or any other suitable material for protection of the components from environmental agents such as water. The choice of the elastomeric potting material can be selected according to the anticipated environmental exposure of the sensor. For example, in certain applications, a low out-gassing material may be appropriate if the sensor is used at high altitude or space while a low compression material may be better if the sensor is used below sea level, such as underwater or underground.
Continuing its description of VRMT sensors, U.S. Pat. No. 7,493,827, further describes that an excitation coil is wound around the poles on one of the magnetic cores, and provides electrical connection for an inductance whose value is variable as a function of the widths of the gaps, and also the axial distortion of the support tube. In the preferred embodiment, there are two excitation coils, each surrounding a separate end of the C-shaped core. This arrangement minimizes non-linearity of response due to fringing effects. The wires from the two coils are twisted and attached to cabling that connects them to external circuitry. Thus, when excited by an external AC voltage, the C-core, the I-core and the gap between the C and I cores form an element of a magnetic circuit. The reluctance of this element is dominated by the gap because the C and I cores are fabricated from high permeability magnetic materials having very little reluctance. The sensor inductance is coupled with a fixed, predetermined capacitance in a resonant inductance-capacitance (LC) circuit. The resonant frequency of the LC circuit is a function of the gap between the C-shaped and I-shaped cores. Accordingly, changes in the gap dimension results in a change in oscillation frequency. Since the only changeable component in the sensor is the number of excitation coils, the sensor is immune to drift.
Finally, as describing VRMT sensor use in an in-line sensor configuration, U.S. Pat. No. 7,493,827 states that, to measure the load on a static device, for example, a chain that moors a marine platform, the support tube is fixedly attached to the surface of a sensor link, and the sensor link placed as a link in the chain. The support tube can be attached to the surface of the sensor link using bolts, by welding, or any other suitable attaching means. In order for the sensor to measure the load on the sensor link, it is preferred that the sensor tube material and the sensor link material are compatible, more preferably the same material or material having the same or similar coefficient of thermal expansion. In the preferred embodiment, the support tube and the sensor link are made of steel. When used in marine applications, it is preferred that a protective coating is applied to the support tube and the sensor link.
Mechanical amplifiers have also seen use in conjunction with VRMT sensors, with one such amplifier configuration being described in prior art U.S. Pat. No. 6,880,408, which is incorporated herein by reference in its entirety. Such an amplifier is described to generally amplify a given load by multiplying the displacement by the stiffness of the load carrying member in order to obtain a reliable displacement measurement. The precision of the measurement as a percentage of full scale load is determined by the ratio of the smallest displacement that the device can resolve to the displacement under full load. In this patent, an embodiment of mechanical amplifiers is described as comprising first and second amplifier mounting pads, at least one of the first and second amplifier mounting pads connecting to a load carrying member, the first and second sensor mounting pads connected to the first and second amplifier mounting pads via flexible connecting members; and a sensor connected to the first and second sensor mounting pads.
Regarding conventional designs of porch-mounted systems, compression load cells have been employed in such systems and as configured they have the sensitivity needed to work in the limited space available between the tendon porch and the tendon top connector assembly. One major drawback with the compression load cells that are often configured in porch-mounted systems is that after only a few years of service they begin providing unreliable measurement signals. This is not advantageous for a floating platform that may have a service life of 20-50 years as it means the compression load cells need to be serviced often over the lifespan of the platform. In addition to this drawback, the compression load cells and their embedded sensors cannot be serviced or replaced without completely removing tension from the associated tendon line. This results in increased service costs and potential downtime for a given platform which incurs additional costs.
One other drawback regarding porch-mounted compression load cell tendon tension monitoring systems results from the arrangement of discrete compression load cells about the central axis of a tendon line along the central axis of a tendon top connector assembly. This results in a discontinuous load path from the upper load plate to the lower load plate. This requires the load plates to be stiff enough to resist significant deflection between the load cell contact points, and also wide enough to transmit the tendon force among the discrete compression load cells. Both of these factors result in heavy and expensive plate sections, which incur significant initial costs and again require additional expenditure to service.
A porch-mounted tendon tension monitoring system is thus desired that has reduced service requirements over time and simpler serviceability when needed. It would also be desirable to optimize and utilize variable reluctance measurement technology sensors in a porch-mounted tendon tension monitoring system.